U.S. patent application number 09/906171 was filed with the patent office on 2002-02-14 for dynamic link assignment in a communication system.
Invention is credited to Nimon, Matthew D., Thesling, William H., Vanderaar, Mark J..
Application Number | 20020018527 09/906171 |
Document ID | / |
Family ID | 22822800 |
Filed Date | 2002-02-14 |
United States Patent
Application |
20020018527 |
Kind Code |
A1 |
Vanderaar, Mark J. ; et
al. |
February 14, 2002 |
Dynamic link assignment in a communication system
Abstract
An architecture for the dynamic assignment of links in a
multi-user communication system. A plurality of information
channels are provided in a forward communication link of the
communication system for carrying channel information of the
plurality of information channels from a transmitter to a plurality
of corresponding receiving devices. The channel information in
corresponding select ones of the plurality of information channels
is varied dynamically in response to link conditions of the
associated receiving devices to more efficiently utilize the
channel bandwidth.
Inventors: |
Vanderaar, Mark J.; (Medina,
OH) ; Nimon, Matthew D.; (Fairview Park, OH) ;
Thesling, William H.; (Bedford, OH) |
Correspondence
Address: |
ARTER & HADDEN, LLP
1100 HUNTINGTON BUILDING
925 EUCLID AVENUE
CLEVELAND
OH
44115-1475
US
|
Family ID: |
22822800 |
Appl. No.: |
09/906171 |
Filed: |
July 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60220261 |
Jul 24, 2000 |
|
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|
Current U.S.
Class: |
375/259 |
Current CPC
Class: |
H04L 1/0025 20130101;
H04L 5/0048 20130101; H04L 27/2657 20130101; H04L 27/2601 20130101;
H04L 27/2602 20130101; H04L 1/0003 20130101; H04L 5/0053 20130101;
H04L 1/0009 20130101; H04L 1/0026 20130101; H04L 2001/0093
20130101; H04L 5/0007 20130101; H04L 5/003 20130101 |
Class at
Publication: |
375/259 |
International
Class: |
H04L 027/00 |
Claims
What is claimed is:
1. A method of communicating data in a multi-user communication
system, comprising the steps of: providing channel information of a
plurality of information channels in a communication link between a
transmitter and one or more receiving devices; varying said channel
information in one or more of said plurality of information
channels in response to associated link conditions between said
transmitter and said corresponding receiving devices; and
transmitting said channel information with said transmitter to said
corresponding receiving devices; wherein said channel information
is received by said respective receiving devices under
substantially all link conditions.
2. The method of claim 1, wherein said plurality of information
channels in the providing step are time division multiplexed.
3. The method of claim 1, wherein said channel information in said
one or more of said plurality of information channels in the
varying step is varied in modulation, coding, gain, and frequency
in accordance with said link conditions of said corresponding
receiving device.
4. The method of claim 1, wherein said channel information in said
one or more of said plurality of information channels in the
varying step is varied in modulation, coding, gain, and frequency
in accordance with said link conditions associated with said
corresponding receiving device such that synchronization of said
respective information channel is maintained.
5. The method of claim 1, wherein synchronization of carrier and
timing between said transmitter and said plurality of receiving
devices is achieved utilizing a central channel of said plurality
of information channels in said communication link.
6. The method of claim 5, wherein said central channel includes a
plurality of information slots such that each said information slot
must contain information in order to achieve said synchronization
of carrier and timing.
7. The method of claim 1, wherein the communication system is a
satellite communication system that transmits said channel
information in said communication link that is a wireless forward
communication link.
8. The method of claim 1, wherein said channel information of each
said plurality of information channels in the providing step
includes a unique ID, said unique ID for uniquely identifying each
said receiving device.
9. The method of claim 8, wherein said unique ID is utilized to
transmit said channel information in unicast to selects one of said
plurality of receiving devices or in multicast to all said
receiving devices.
10. The method of claim 1, wherein said channel information in the
varying step is varied based upon the combination of modulation and
coding in accordance with said link conditions of said
corresponding information channel.
11. The method of claim 1, wherein the varying step is performed on
select ones of said plurality of information channels utilizing
time division multiplexing and orthogonal frequency division
multiplexing.
12. An apparatus for communicating data in a multi-user
communication system, comprising: a plurality of information
channels in a communication link of the communication system for
carrying channel between a transmitter and one or more receiving
devices; and a varying device for varying said channel information
in one or more of said plurality of information channels in
response to associated link conditions between said transmitter and
said corresponding receiving devices; wherein said channel
information is transmitted with said transmitter to said
corresponding receiving devices; wherein said channel information
is received by said respective receiving devices under
substantially all link conditions.
13. The apparatus of claim 12, wherein said plurality of
information channels are time division multiplexed.
14. The apparatus of claim 12, wherein said channel information is
varied in modulation, coding, gain, and frequency in accordance
with said link conditions of said corresponding receiving
device.
15. The apparatus of claim 12, wherein said channel information is
varied in modulation, coding, gain, and frequency in accordance
with said link conditions associated with said corresponding
receiving device such that synchronization of said respective
information channel is maintained.
16. The apparatus of claim 12, wherein synchronization between said
transmitter and said plurality of receiving devices is achieved
utilizing a central channel of said plurality of information
channels in said communication link.
17. The apparatus of claim 16, wherein said central channel
includes a plurality of information slots such that each said
information slot must contain information in order to achieve said
synchronization, said synchronization including both carrier and
timing.
18. The apparatus of claim 12, wherein the communication system is
a satellite communication system that transmits said channel
information in said communication link that is a wireless forward
communication link.
19. The apparatus of claim 12, wherein said channel information of
each said plurality of information channels includes a unique ID,
said unique ID for uniquely identifying each said receiving
device.
20. The apparatus of claim 19, wherein said unique ID is utilized
to transmit said channel information in unicast to select ones of
said plurality of receiving devices or in multicast to all said
receiving devices.
21. The apparatus of claim 12, wherein said channel information is
varied based upon the combination of modulation and coding in
accordance with said link conditions of said corresponding
information channel.
22. The apparatus of claim 12, wherein said channel information is
varied on select ones of said plurality of information channels
utilizing time division multiplexing and orthogonal frequency
division multiplexing.
Description
[0001] This application claims priority under 35 U.S.C. .sctn.
119(e) from U.S. Provisional Patent application Serial No.
60/220,261 entitled "Dynamic Link Assignment" and filed Jul. 24,
2000.
BACKGROUND OF THE INVENTION
[0002] 1. Technical Field of the Invention
[0003] This invention is related to digital communication systems,
and more particularly to multi-user satellite systems for providing
user access to a global packet-switched data network.
[0004] 2. Background of the Art
[0005] The advent of the Internet and the commercial opportunities
offered by reaching the millions of potentially new customers which
connect thereto have motivated some companies to provide wireless
connectivity for those users which cannot use conventional means
hardwired means. For example, satellite-based systems provide a
mechanism whereby users who are only offered conventional
relatively low modem speed access or have no alternative for
connecting at all, can now connect to such packet-based systems at
higher speeds.
[0006] However, inefficient use of resources in multi-user
satellite systems results in excessive link margins that
drastically reduce system capacity. Typically, the forward link
from the satellite to the user is a time-multiplexed data stream
that is received by a large number of user terminals. As such, the
satellite must be capable of providing service to the user that is
under the lowest quality link conditions. Existing satellite
communication packet-based systems which offer access to the
Internet can transmit digital information to users in unicast, that
is, the digital information can be sent to a specific user based
upon a unique identification number (ID) assigned to that user, the
unique user ID derived via any number of conventional methods.
However, existing unicast transmissions still fail to efficiently
utilize the available bandwidth by formatting and sending the
unicast data under constraints, which anticipate the worst possible
reception conditions for any user to reasonably ensure that all
users can receive the transmission. This "one-size-fits-all"
problem requires satellite systems to operate with link margin
requirements that are extremely wasteful to system capacity.
[0007] What is needed is a link architecture that allows the link
to be customized on a per-user basis to more efficiently utilize
channel bandwidth in the communication system.
SUMMARY OF THE INVENTION
[0008] The present invention disclosed and claimed herein, in one
aspect thereof, comprises architecture for the dynamic assignment
of links in a multi-user communication system. A plurality of
information channels are provided in a forward communication link
of the communication system for carrying channel information of the
plurality of information channels from a transmitter to a plurality
of corresponding receiving devices. The channel information in
corresponding select ones of the plurality of information channels
is varied dynamically in response to link conditions of the
associated receiving devices to more efficiently utilize the
channel bandwidth.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] For a more complete understanding of the present invention
and the advantages thereof, reference is now made to the following
description taken in conjunction with the accompanying drawings in
which:
[0010] FIG. 1 illustrates a block diagram of a frequency
channelization scheme, in accordance with a disclosed
embodiment;
[0011] FIG. 2 illustrates a flow chart of the process for
dynamically controlling a user link in accordance with present link
conditions;
[0012] FIG. 3 illustrates a graph of an OFDM waveform and channel
numbering scheme based around a center frequency;
[0013] FIG. 4 illustrates a graph of frequency response of a
simulated dynamic link assignment waveform;
[0014] FIG. 5 illustrates organization of the various slots
utilized in a frame;
[0015] FIG. 6 illustrates a diagram of a Synchronization slot;
[0016] FIG. 7 illustrates a frame structure of a Receiver Access
Channel slot;
[0017] FIG. 8 illustrates a diagram of a Frame Definition slot;
[0018] FIG. 9 illustrates a diagram of a receiver User/Message
Definition slot; and
[0019] FIG. 10 illustrates an example of the channel/slot structure
of the dynamic link assignment architecture.
DETAILED DESCRIPTION OF THE INVENTION
[0020] The disclosed Dynamic Link Assignment (DLA) architecture
provides the capability of more than quadrupling channel capacity
in a multi-channel system by presenting a multi-user access scheme
that allows the communication system to dynamically customize,
without requiring resynchronization and associated loss of data, a
user waveform to match the user link conditions.
[0021] In a satellite-based application, the architecture allows
variable modulation and coding formats on a per-user basis through
the use of Time Division Multiplexing (TDM) and Orthogonal
Frequency Division Multiplexing (OFDM). A user terminal provides
feedback to the satellite system such that the forward link to the
user can be customized dynamically according to link conditions at
any particular moment. Moreover, as the OFDM waveform is frequency
and time locked, a user can change modulation and coding rapidly
without resynchronization. Carrier and timing synchronization is
achieved on a central, data-bearing channel. This arrangement
allows the overall forward link to be customized on a per-user
basis, allowing for reduced operating margin. Additionally, a
combination of modulation and turbo coding provides bandwidth and
power efficiency that approach Shannon's limit. Although the
following discussion focuses on satellite-based systems, the
disclosed architecture is not restricted to satellite systems, but
has application in any multi-user digital communication system in
which data transmission is to a number of users each operating
under different conditions, e.g., a passive optical network.
[0022] Referring now to FIG. 1, there is illustrated a general
block diagram of a channelization scheme 100, in accordance with a
disclosed embodiment. In OFDM, a subcarrier pulse 102 used for
transmission of information is chosen to be rectangular, which
shape has the advantage that pulse forming and modulation at the
head-end transmitter can be performed by an Inverse Discrete
Fourier Transform (IDFT) that can be implemented very efficiently
as an Inverse Fast Fourier Transform (IFFT). Accordingly, at the
receiver, an FFT is needed to reverse (or demultiplex) the
channels. Leading and trailing guard bands 104 are used to combat
multipath signals.
[0023] In general, the overall bandwidth per primary channel 106 is
approximately x MHz, and each primary channel 106 is subdivided
into n sub-channels S (denoted 108, and where n=0, . . . , p), that
overlap in an OFDM sense, resulting in a symbol rate of x/n
M-symbols/sec (Msps) sub-channel. Within each sub-channel S.sub.n
108, a frame structure is defined (and is discussed in greater
detail hereinbelow) such that there are 2.sup.z symbols per frame,
where z is selected for optimal signal quality. The channel
numbering scheme is based around a center frequency fc (denoted
110), such that a first sub-channel S.sub.0 112 is centered at the
center frequency 110. The remaining sub-channels 108 are
distributed about the center frequency 110 as illustrated in FIG.
1.
[0024] Referring now to FIG. 2, there is illustrated a flow chart
of the process for dynamically controlling a user link in
accordance with present link conditions. Flow begins at a starting
point 200 and continues to a function block 202 where the user
terminal (or ground-based terminal wherever it may be located)
determines the current channel transmission parameters based upon
the existing link conditions for that user location. Flow is to a
decision block 204 to then determine if link conditions for that
user channel have changed. If not, flow is out the "N" path to a
function block 206 to maintain the current channel parameters for
that user. Flow is then back to the input of function block 202
where the user terminal again determines the link conditions. On
the other hand, if the link conditions for that channel have
changed, flow is out the "Y" path of decision block 204 to a
function block 208 where the satellite hub receives the current
link parameters for that channel as a link status signal. The hub
then adjusts the signal channel for optimum operating parameters
according to current link conditions, as indicated in a function
block 210. Flow is then to a function block 212 to transmit the
user channel information to the user terminal under the adjusted
channel parameters.
[0025] The channel adjustment process is performed dynamically in
response to existing link conditions for that particular user
terminal. It can be appreciated that in a power-up scenario, or
where the link between the user terminal and satellite hub is lost,
a synchronization process occurs under default operating condition
to establish the link as soon as possible. To that end, a feedback
path exists between the user terminal and satellite hub wherein the
link conditions for that particular user are being continually
monitored such that the forward link for that user channel can be
adjusted to ensure optimum channel quality under existing link
conditions. The return path from the ground-based user terminal to
the satellite can be a direct wireless path from the user
transmitter (e.g., a satellite dish system) to the satellite hub.
Alternatively, the link from the user terminal can be via other
conventional means such as a return path through a telephone line
to an access provider who then completes the return link to the
satellite hub. Other methods for providing the return path from the
user to the satellite are commonly known by those skilled in the
art.
Waveform Description
[0026] Referring now to FIG. 3, there is illustrated a graph of an
OFDM waveform 300 and channel numbering scheme based around the
center frequency fc 110, in accordance with a disclosed embodiment.
Note that channel zero 112 is defined as the channel that is
centered on the center frequency 110. The bandwidth 302 of the main
lobe 106 is nominally 54 MHz with a null-to-null bandwidth 304 of
57.375 MHz. In the unfiltered case, the first side lobes 306 are
approximately 18 dB down (i.e., -18 dB) from the main lobe. In
order to maintain accurate synchronization, the DLA waveform is
constrained to require a special waveform in the central channel.
The central channel 112 is received at the baseband, and uses a
special waveform in order to maintain synchronization. The waveform
in channel zero 112 consists of QPSK (Quadrature Phase Shift Key)
data (no constraint on coding or gain), with some side information
to aide in synchronization. Information must be present in all
channel-zero 112 slots. In cases where the channels 108 do not fit
"evenly" into the primary band 106, a partial channel (not shown)
is supported. For partial DLA channels, channel zero 112 must be
present.
[0027] Referring now to FIG. 4, there is illustrated a graph 400 of
the frequency response of a simulated DLA waveform in the example
of FIG. 3. The main lobe 106 has a bandwidth of approximately 54
MHz with the first side lobes 306 down approximately 18 dB from the
main lobe 106.
Framing Description
[0028] Referring now to FIG. 5, there is illustrated the
channelization frame structure. The DLA architecture provides a
number of slot and packet types for use within the frame 500 to
allow users to enter and exit the transmission system, and to
provide the customized user link. The DLA slot types include one or
more of the following: a Synchronization slot, a Receive Access
Channel (RAC) slot, a Frame Definition State (FDS) slot, and DLA
User/Message (U/M) slot.
[0029] The Synchronization slot appears as the first slot 502 once
per frame 500 to allow reliable modem synchronization. The RAC slot
is in the second slot 504, and is a reliable slot that contains
user ID tables to allow users to enter the transmission system for
data reception in the current frame 500. The entry information for
both single-user IDs and broadcast/multicast IDs are supported in
the RAC slot 504. In addition to system entry, the RAC slot 504
provides for a low-latency hardware-messaging path. Two FDS slots
506 and 508 contain information regarding the location (in time and
frequency) of slots in the next frame, and the format (modulation,
coding, and gain) of user slots in the current frame 500. The FDS
slots 506 and 508 appear as the third and fourth slots on each
frequency sub-channel 108. A number of U/M slots 510 (U/M1, . . . ,
U/Mn) contain the user transport stream payload, and comprise two
classes of user slots: a single user per slot and a multi-user slot
to handle low data rate traffic such as voice. The single user slot
may be directed toward an individual terminal, or may be a
broadcast or multicast slot as originally defined by the RAC slot
504.
DLA Synchronization Slot
[0030] Referring now to FIG. 6, there is illustrated a diagram of a
synchronization frame 600. The synchronization frame 600 appears as
the first slot 502 of each channel 108 and each frame 500, and
consists of a preamble filed 602 that contains BPSK (Binary Phase
Shift Key) ones for 3,841 symbols, the utilization of which allows
the terminal demodulator to acquire the carrier frequency and
phase, as well as the symbol timing.
[0031] Following the preamble field 602 is a Unique Word (UW) field
604 that signifies the beginning of the frame 500. The UW field 604
consists of 255 BPSK symbols, and is generated via an 8-bit linear
feedback shift register with a polynomial value of
x.sup.8+x.sup.4+x.sup.3+x.sup.2+1, and a seed value of 0.times.10.
The UW frame 600 is sufficient for reliable frame detection at a
channel Signal-to-Noise Ratio (SNR) that corresponds to the most
power efficient modulation and coding, specifically an SNR=-3.0 dB.
A hard decision parallel correlator with a programmable threshold
is the preferred approach for acquiring frame synchronization.
DLA Receiver Access Channel Slot
[0032] Referring now to FIG. 7, there is illustrated a structure of
a Receiver Access Channel frame 700. The RAC frame 700 contains
information that allows users to enter the transmission system or
receive messages based upon the user ID or broadcast ID. The
following constraints are placed on the RAC frame 700: (1) data in
RAC frame 700 that is repeated across sub-channels 108 is rotated
from sub-channel to sub-channel to prevent a power surge in the DLA
link, and (2) broadcast and multicast ID information must only
occur in the sub-channel 108 that is equivalent to the upper four
bits of the broadcast/multicast ID. This provides for ease of use
and entry into broadcast/multicast data streams.
[0033] Starting in slot two 504 of every frame 500, each RAC frame
700 contains a set of individual user IDs and a smaller set of
broadcast IDs. The RAC frame 700 contains 4,096 QPSK symbols
encoded with two code blocks 702 and 704 of (4096, 1331) TPC (Turbo
Product Code) data, each having a set of 1,331 information bits
(706 and 708), totaling 2,662 information bits, and each having
2,765 corresponding code bits (710 and 712). This allows for forty
user IDs (Users 0-39) in each RAC frame 700, or 128 new users per
second.
[0034] Each set of information bits 706 (and 708) contains a 16-bit
RAC Header 714 which is the first sixteen bits of each TPC block
702 and 704. The first eight bits of the header 714 indicate a
frame counter 715, and the next eight spare bits 717 of the header
714 are reserved for future use. There are twenty User fields 716
(User 0-19) per set of information bits 706 (and 708), and each
User field 716 contains sixty-four bits: a 48-bit User ID 718, an
8-bit Control field 720, and an 8-bit Data field 722. Each of the
User fields 716 contains information for an individual user,
multicast users, or broadcast users. The 48-bit User ID (or
Broadcast ID) field 718 conforms to the IEEE 802.3 standard. Each
user, broadcast, and multicast is uniquely identified by the User
ID 718 or physical MAC (Media Access Control) address. The
broadcast and multicast IDs are made available to registered users
and stored in a data file on the terminal computer. The four
most-significant bits of the broadcast and multicast IDs correspond
to the channel on which the broadcast is transmitted. The Control
byte field 720 is a control command, and is discussed in greater
detail with respect to messaging. The primary purpose of the Data
byte 722 is to identify the slot number in which the user data or
message appears in the current frame. However, for certain control
commands, the Data field 722 can contain other data, which is
discussed in greater detail hereinbelow with respect to messaging.
There are two 32-bit CRC (Cyclic Redundancy Check) fields 724, one
for each set of information bits 706 and 708 which provide error
detection for the header 714 and the twenty user information
packets 716, in their respective TPC blocks 702 and 704. There are
also two 3-bit zero pad fields 719, one for each set of information
bits 706 and 70-8 which serve to fill out the TPC blocks.
DLA Frame Definition State Slot
[0035] Referring now to FIG. 8, there is illustrated a diagram of
the third and fourth slots 506 and 508, the Frame Definition State
slots. The third and fourth slots (506 and 508, respectively) in
each channel are the FDS slots, and each contains the modulation,
coding, gain, next channel, and next slot information for each user
within a channel. Each FDS slot 506 and 508 contains 4,096 QPSK
symbols, and each set of 4,096 QPSK symbols corresponds to two
blocks of (4096, 1331) TPC coded data. A first TPC block 800 of the
first FDS frame 506 contains 1,331 information bits 810 and 2,765
corresponding code bits 812. A second TPC block 804 of the first
FDS frame 506 contains 1,331 information bits 814 and 2,765
corresponding code bits 816. A first TPC block 806 of the second
FDS frame 508 contains 1,331 information bits 818 and 2,765
corresponding code bits 820. A second TPC block 808 of the second
FDS frame 508 contains 1,331 information bits 822 and 2,765
corresponding code bits 824. This provides 2,662 information bits
for each of the two FDS slots 506 and 508, for a total of 5,324
information bits.
[0036] Each set of information bits (810, 814, 818 and 822) further
subdivides into sixty-four 20-bit Slot Definition fields which
contain information about user slots [4 . . . 255], a Spare bits
field 828 of sixteen spare bits, a 32-bit CRC field 830 for error
detection over the previous sixteen spare bit fields 828,
sixty-four slot definition fields 826, and a 3-bit zero pad field
831. The CRC field 724 adds an additional layer of error checking
to prevent spurious jumps from frame to frame. Information for
slots [0 . . . 3] provide default settings. Each 20-bit Slot
Definition field 826 is divided into the following three subfields:
an 8-bit Modulation, Coding, and Gain field 832 which specifies the
modulation, TPC coding, and gain format of the user slot in the
current frame (the default value in slots [0 . . . 3] is
0.times.01) (the 8-bit value is extracted by the terminal and
decoded to three distinct configuration values that are used by the
terminal to set-up the user slots); a 4-bit Next Channel field 834
that indicates which channel the user slot will use in the next
frame (the default value in slots [0 . . . 3] is 0.times.00); and
an 8-bit Next Slot field 836 that indicates which time slot the
user slot will use in the next frame. If the Next Channel field 834
and Next Slot field 836 point to the primary RAC channel, the user
goes to the RAC in the next frame.
DLA Receiver User/Message (U/M) Slots
[0037] Referring now to FIG. 9, there is illustrated a diagram of a
user definition slot. The transport stream appears at the user slot
level and is based upon a custom transport stream structure. The
DLA transport stream structure varies based on the combination of
modulation and TPC coding used on the channel. Each U/M slot 900
contains one or more TPC blocks 902. FIG. 9 illustrates a U/M slot
900 containing four TPC blocks 902. Each TPC block 902 contains a
standard Header 904, Payload data 906, a CRC 908, and Parity bits
910. The 32-bit Header field 904 contains header information for
the user slot, which user slot information is described using three
sub-fields: an 11-bit Start-of-Protocol Packet pointer 912 which is
used to point to a byte location in the payload filed which is the
first byte of a higher layer protocol packet (IP, for example), and
if no start-of-packet occurs in the TPC block, this protocol
pointer field 912 is set to 0.times.7FF; a 13-bit Length Field 914
which identifies the length (in bytes) of the information payload
906 (and is used by the device driver to determine what data to
pass to the higher layers in the protocol stack); and an 8-bit Next
Slot Number filed 916 which identifies the next valid U/M slot for
the user in the current frame. If it is the last slot for the user
in the particular frame, this value is set to 0.times.00.
[0038] The size of the Payload field 906 ranges from 644 to 15,208
bits. This variable-length field 906 contains the payload that is
used for transporting data or messages. Software ensures that the
length of valid data in the payload field 906 is always an integral
number of bytes. The 32-bit CRC field 908 provides error detection
for the header 904 and payload 906 of the slot 900. The Parity
field 910 is a variable-length field, which contains the TPC parity
bits.
[0039] Referring now to FIG. 10, there is illustrated an example of
the channel/slot structure of the dynamic link assignment
architecture. Each 54 MHz primary band 106 contains sixteen
frequency-multiplexed sub-channels 108 that partially overlap in an
OFDM fashion, providing bandwidth efficiency near Nyquist
requirements. In this particular embodiment, the sixteen
sub-channels 108 are modulated in the main lobe 106 of the
subcarrier pulse 102. Each sub-channel 108 operates at {fraction
(1/16)}.sup.th of the 54 MHz channel frequency providing a symbol
rate of {fraction (54/16)}=3.375 Msps. The nominal capacity
C.sub.nom of the x=54 MHz primary channel is calculated assuming a
nominal modulation and coding that yields 2.5 bits/symbol. The
nominal capacity C.sub.nom is calculated as follows:
C.sub.nom=(2.5 bits/symbol)(3.375 Msps/channel)(16
channels/composite signal)=135 Mbps.
[0040] Each sub-channel frame 1000 is structured to facilitate the
disclosed link architecture. For example, in a channel 1002, the
corresponding frame 1004 (for a single frame period of 0.311
seconds) begins with a synchronization frame 1006, followed by a
RAC slot 1008, two FDS slots 1010 and 1012, and multiple user slots
1014. All other channels have the similar frame structure.
[0041] Note that the disclosed architecture can be implemented in
hardware such that one or more digital devices are fabricated to
provide a high speed solution (e.g., digital CMOS chip).
[0042] The disclosed architecture, in general, has application in
any point to multi-point digital communications link in which the
"multi-points" have different link conditions and feedback is
provided to monitor and control the link in response to changing
link conditions. For example, an application includes a cellular
telephone that uses a point (base station) to multi-point (cell
phones) configuration under various link conditions (e.g., antenna
size, receiver sensitivity, interference, distance to base station,
etc.).
[0043] The invention also has application where the overall system
architecture includes a multi-point to multi-point configuration,
as long as it can be decomposed into at least one point to
multi-point link.
[0044] Although the preferred embodiment has been described in
detail, it should be understood that various changes, substitutions
and alterations can be made therein without departing from the
spirit and scope of the invention as defined by the appended
claims.
* * * * *